The successful integration of an implant in a patient is dependent on many factors including the extent of integration of the implant into the body, the prevention of infection, and proper material compatibility.
Historically, an orthopaedic fixation was applied to stabilize fractures and maintain the reduction of a deformity. For example, the earliest fixation methods involved the use of loops of silver wire, which were passed around the spinous process to immobilize the spine in cases of tuberculosis. Later, attempts were made to wire supporting rods made of synthetic material and/or iron to the spine to maintain stabilization. But, because these were ferrous materials, electrolytic reactions occurred, infections developed, and the results were generally unsatisfactory.
Over the years there has been an evolution to the use of different materials for stabilization, internal splintage and fixation. For example, by the 1930s Venable and Stuck, two orthopedists in Texas, identified that the use of an orthodontic alloy called Vitallium was very suitable in orthopaedics. (See, Venable C. S., et al., Ann Surg. 105, 917-938 (1937), the disclosure of which is incorporated herein by reference.) The material was unreactive with the tissues and indeed this stainless steel alloy was the main material for internal fixation and stabilization for the next sixty years. However, because of changing imaging technologies, stainless steel alloys, which produced greater artifacts during the imaging process, have been replaced by other materials, such as titanium. In addition, stainless steel alloys always had the complication that they were surrounded by a fibrous scar, which encapsulated the device. Titanium, on the other hand, functions more like a ceramic material, in that bone actually grows into the interstices of the crystalline lattice structure of the material producing superior fixation.
However, some of the physical properties of titanium are not as desirable as stainless steel. For example, stainless steel alloys have a very high rigidity-titanium of equal size has only 54% of the rigidity of stainless steel. In addition, the yield strength of titanium, i.e., the load required to statically deform the internal crystalline structure of the material is 2.34 times greater in commercially pure titanium than in stainless steel alloys, which results in some contoured implants straightening out after they have been contoured to match the body. The difference between the ultimate strengths of the two materials, titanium is less rigid than stainless steel having only half the rigidity of most stainless steel alloys.
In addition to the implantation concerns, another area of concern is infection caused by the introduction of the implant into the patient or the use of an implant in a clinical setting where increased rate of infection in immuno-suppressed patients is prevalent. The treatment of infected implants is quite controversial. There are those who feel that the removal of the implant is the only way to eradicate the infection. However, there are others who feel that the removal of the implant promotes instability, condition.
For example, often it is found that the implant becomes problematic because bacteria hide in the interstices of the crystalline structure of the metal or nonmetal implant. This makes the eradication of infection difficult. On the other hand, the clinical challenge is that if the internal stabilizing system is removed, the deformity can recur and stability may be lost, which can effect neurological and vascular function and/or result in a great increase in the patient's pain and discomfort. Moreover, the incidence of chronic infection in the United States is increasing as more and more antibiotic resistant bacteria are spreading through hospitals, extended care facilities and the community.
As a result, all too often patients have delayed recovery because of infection, implants are removed, and patients are treated with the implant removed. Because of the enormous surgical and clinical complications that can arise from such drastic revision surgery, it is often the case that patients are faced with a less than perfect clinical outcome.
One final complication that can arise is an adverse interaction between one or more components of the implants. For example, it can often be the case that orthopaedic implant systems would be made more suitable by the use of a variety of metal materials in the different components. However, galvanic interaction between the metal types can lead to serious degradation of the implant. As a result, typically a single less than perfect material is used for all of the components of the implant system to ensure against such adverse interactions.
In short, for vertebral stabilization systems, such as bone/pedicle screws, the integration of the implant is dependent on the development of a healthy screw bone interface at a macroscopic and microscopic level. Also, because metal is rigid and unyielding and bone is a living cellular structure with a crystalline scaffolding, as the metal component compresses it can distort the crystalline lattice and the cellular component of bone leading to bone implant interface loosening. However, while all patients face these challenges, there are special circumstances where patients, in the course of their life, have developed decreased bone mineralization. The crystalline lattice is weaker. There is less cellular response, and therefore, fixation between bone and metal is less satisfactory.
An exemplary group of patients that fall within this at-risk category are elderly patients who have osteoporosis, osteopenia or osteomalacia. These patients are a challenge for fixation systems in orthopaedics because the structural strength of the bone has been diminished by biological or pathological processes, i.e., osteopenia, ovariectomized patients, postmenopausal patients, patients with low serum testosterone, and patients who have suffered from radiation, etc., and other cases of medically induced osteopenia.
It is these special groups of patients who most often have orthopaedic interventions that are fraught with complications, ultimately producing fixation failure. One way to avoid such outcomes would be to develop improved coating systems that could address fixation, infection, and material interaction. Unfortunately, to-date such coating systems have not been applied to bone/pedicle screws or any of the associated hardware.